Dozens of householders were told they would save money by switching to the Worcester-based provider, when in reality they would be worse off, according to a report in a national newspaper.

The alleged underhand tactics highlighted included exploiting people with poor English, pretending to be ‘electricity board’ officials, lying about standing charges and getting people to sign a form without revealing it was actually a contract.

A dossier was presented to senior npower executives on Friday, after which the energy firm suspended a team of 17 salespeople operating in London.

Peter Luff, chairman of the Commons business and enterprise committee, has called for Ofgem, the industry regulator, to investigate and also plans to conduct his own inquiry.

Mr Luff, MP for Mid Worcestershire, saidthis in sunday times report: "This seems to be further worrying evidence of possible failures in the UK utilities market and we will pay careful attention to this."

Npower posted record profits of £57 million last year. It signed up 170,000 new customers despite charging some of the highest energy prices with average increases of 12.7 per cent for electricity and 17.2 per cent for gas this year.

Among the vulnerable people allegedly exploited by npower’s staff were a 91-year-old blind man and a man with learning difficulties. When their carers complained both salesmen were suspended.

Richard Frost, spokesman for npower, said: "There is a certain pride in what we do, so these very serious allegations are of huge concern to us.

"We’re not always the cheapest provider but we do have some attractive offers to customers, so there should be absolutely no need to use underhand tactics to make a sale or get a customer to switch companies.

"If we find that any of our sales staff have acted fraudulently they will be dismissed, there’s no doubt about that.

"An accreditation scheme called EnergySure operates across the energy sales industry but if salespeople fail to behave properly that accreditation can be removed. Normally we would not reveal the details of any such investigation, but we recognise there’s a public interest issue to address here."